(i) It is a simple rule to predict qualitatively the relative stability of acid - base reactions.
(ii) "Hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases". Thus the complex A : B is the most stable when both A and B are either hard or soft.
(iii) However, when one of the reactants is very hard and the other very soft, the complex will not be much stable.
(iv) Hard-hard interactions involve ionic bonding and soft-soft interactions result primarily from covalent bonding.
(v) Soft-soft interaction can be explained on the basis of π-bonding. Soft acids have low oxidation states and possess large number of d-electrons. Thus they can act as π-bond donors and soft bases are generally π-bond acceptors. The presence of d-orbitals on the ligands (except on CO) helps to strengthen the π-bonding.
